There have conventionally been proposed various types of video display device that employ a holographic optical element (hereinafter also referred to as HOE) to allow an observer to observe as a virtual image the image displayed on a display element. For example, in the video display device disclosed in Patent Document 1 listed below, a plurality of HOEs corresponding to a plurality of display elements are arranged in an array on the same plane so that the image light from those display elements are diffraction-reflected by the corresponding HOEs so as to be directed to the observer's pupil. This permits the observer to observe a large high-definition image.
On the other hand, in the video display device disclosed in Patent Document 2 listed below, in a configuration where the image light from a display element is diffraction-reflected by an HOE so as to be directed to the observer's pupil, the HOE is formed in a cylindrical shape. That is, the HOE is formed on a cylindrically shaped substrate, and thus the HOE surface as a whole is a cylindrical surface having a constant curvature.
Inconveniently, with the configuration of Patent Document 1, where a plurality of HOEs are arranged in an array on the same plane, as the observation angle of view increases, the diffraction angle of the image light on the HOE varies greatly within the image area. That is, the diffraction angle on the HOE (in particular, the difference between the diffraction angle and the regular reflection angle) becomes larger in end parts of the angle of view than at the center of the angle of view. As a result, with a configuration where a light source, such as an LED, that emits light with a light emission wavelength width is used and a display element modulates the light from that light source to display an image, dispersion on the HOE produces large lateral chromatic aberration in a part where the diffraction angle is large (in end parts of the angle of view). This diminishes resolution and degrades image quality.
In this respect, according to Patent Document 2, forming the HOE surface as a cylindrical surface makes it possible to reduce the diffraction angle (make it closer to the regular reflection angle) in end parts of the angle of view even in a wide-angle-of-view configuration. Thus, it is possible to reduce the above-mentioned aberration occurring when the HOE surface is a flat surface, and to allow observation of a satisfactory virtual image.